The present invention relates to a circuit arrangement for receiving unipolar direct current signals transmitted via a transmission line of a telecommunication system such as a teleprinter system or a data transmission system. In the circuit arrangement there is provided an opto-coupler circuit for non-conductive connection of the receiving branch of the transmission line with an evaluation circuit having adjustable threshold value characteristics for evaluating the incoming DC signals, thereby compensating distortions. The term `opto-coupler` as used throughout the specification and claims is intended to be synonymous with `optoelectronic coupler` and/or `electrooptical isolator`, the latter terms being in more prevalent use in the United States.
Upon transmission of DC signals over a transmission line, the incoming signals have to be evaluated or rated at the receiving end of the line to eliminate low-amplitude interference or noise. Such evaluation is achieved by circuit arrangements often having adjustable threshold values. The evaluation takes into account generally differing time constants for leading and trailing edges of transmitted signals and compensates for distortions resulting from variable signal amplitudes. The requirement that the threshold value be adjustable can be achieved by providing an auxiliary reference voltage for setting this value. However, employing an auxiliary voltage gives rise to the problem of additional influences of the transmission line, which influences can be avoided by use of a floating voltage, i.e. an off-ground auxiliary voltage.
Such design of an evaluation circuit arranged at the receiving end of the transmission line results in a relatively large outlay, but this outlay can be reduced by providing opto-coupler circuits to effect a non-conductive connection of the transmission line with receiver equipment including the evaluation circuit. A circuit arrangement of this type has been described in the British Patent Specification No. 1,427,575. This known circuit arrangement for evaluating DC data signals makes use of opto-coupler circuits performing the non-conductive connection and is based upon all the requirements and measures described.
Whereas the known circuit arrangement fulfills to a large extent the requirements to which receiving circuits are subject, in designing such a circuit arrangement the specific characteristics of opto-coupler circuits have to be taken into consideration. Opto-coupler circuits can be used on the premise of sufficiently accurate operating characteristics but this premise cannot be guaranteed in every case. The forward voltage of a light-emitting diode employed as the input stage of an opto-coupler circuit is subject to change in dependence upon the amplitude of the forward current and temperature. Furthermore, an opto-coupler of the transistor-coupler type presents a non-linear transmission factor, i.e. a transmission factor which varies in dependence upon the diode forward current. These properties, which are generally referred to as non-linearities of coupling circuits, lead to distortions and restrict the use of the known circuit.
It is an object of the present invention to provide an improved circuit arrangement for receiving DC signals transmitted via a transmission line.
Another object of the invention is to provide such a circuit arrangement of a design which retains the recently described properties of the receiving circuitry and avoids distortions associated with non-linearities of opto-couplers used as non-conductive transmitting circuits.